<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <title>块级元素</title>
    <style>
        input {
            color: red;
        }

        :-moz-placeholder { /* Mozilla Firefox 4 to 18 */
            color: #000; opacity:1;
        }

        ::-moz-placeholder { /* Mozilla Firefox 19+ */
            color: #000;opacity:1;
        }

        input:-ms-input-placeholder{
            color: #000;opacity:1;
        }

        input::-webkit-input-placeholder{
            color: #0f0;opacity:1;
        }
    </style>
</head>
<body>

<input type="text" placeholder="请输入用户名">

</body>
</html>